The present invention relates to procedures for analyzing the multi-element atomic composition of molecular compounds and, in particular, to the use of inductively coupled plasma torch optical emission spectrometry (ICP-OES) for this purpose.
As may be known, ICP arrangements usually employ an RF transmitter supplying power to a load coil wound on the torch to energize a plasma or auxiliary gas, such as argon. The energy produces a high temperature plasma environment into which a sample mixed with a carrier gas is introduced. Coolant argon also is used. The high temperature environment thermally degrades the molecular sample and the resulting atomic species are excited to produce radiation emissions detected by optical spectrometric procedures.
In the past, another commonly-used procedure has been the familiar flame photometry from which, in fact, the present ICP procedure essentially is an outgrowth or extension. Also, important analytical work has been conducted with the use of the so-called microwave-excited plasmas (MEPs). However, for one reason or another, these procedures are found to be somewhat limited in their applications. In particular, problems arise when they are applied to the elemental analyses of organic compounds. Flame photometry, for example, is fundamentally inconsistent with organic compound evaluations since the hydrogen in organics also is present in the gas normally used to support the flame.
With regard to MEP, it is recognized that a number of investigators have reported its use for the elemental analysis of organic compounds. However, certain problems have been noted. For example, the emission intensity of an element often is found to be dependent on the structure of the organic molecular compound and the wavelength of maximum intensity for a given element is not consistently the same for all compounds containing that element. Also, there is a problem regarding the recombination of atoms to form diatomic species which, in fact, occurs to such an extent that molecular band heads sometimes are employed for analysis in several MEP systems. A further difficulty is that carbon-, sulfur-, and phosphorus-containing compounds form deposits on the quartz excitation cells of the MEP. In fact, the dependence of carbon response per unit weight on the carbon number of the compound has been attributed to the formation of carbon deposits inside the MEP cell. Molecular oxygen or nitrogen added to the microwave support gas has helped reduce these formations.
In some contrast to the use of MEP, relatively little work has been reported on the use of ICP-OES for the anaylsis of elements in organic compounds. In particular, Fassel, et al have determined wear metals in oils, (Ref, 1, V. A. Fassel, et al, Anal. Chem. 48,516 (1976). Ward reports the determination of metals in organic solvents (Ref. 2, A. F. Ward, ICP Information Newsletter 1, 266 (1976). Nishimura studied the decomposition of gaseous hydrocarbons but was unable to maintain a stable plasma when the organic sample is introduced in a conventional manner (Ref. 3, Y. Nishimura, ICP Information Newsletter 1, 126 (1975). Greenfield and Smith analyzed blood plasma for phosphorus and silicon (Ref. 4, S. Greenfield, et al, Anal. Chim. Acta 59, 341 (1972), and Kniseley, et al determined phosphorus along with several metallic elements in whole blood (Ref. 5, R. N. Kniseley, et al, Clin. Chem. 19,807 (1973). Several non-metallic elements present in organic compounds also have been determined by ICP-OES as inorganic ions in aqueous solutions and a detection limit of 0.1 ppm for phosphorus in diluted oils has been reported (Ref. 6, F. Breach, ICP Information Newsletter 1, 171 (1976) from a Jarrell-Ash Application Note). In summary, these reports, as well as others, are of considerable interest in that they indicate the applicability of ICP-EOS to the multi-element detection of a rather wide variety of elements. However, as far as is known, this technique has not been successfully used for the elemental analysis of organic compounds which is one of the major objects of the present invention. Another very important object is to utilize the ICP to determine the actual empirical formula of an organic compound or other similar compounds. Again, the prior art apparently has not considered this particular aspect even though its achievement should be of considerable benefit.